Artificially coated NaFePO4 for aqueous rechargeable sodium-ion batteries

Abstract Aqueous rechargeable sodium-ion batteries are attractive as alternative materials to replace conventional lithium-ion batteries for the development of next-generation devices due to the abundance of sodium resources. Among various positive electrode materials for sodium-ion batteries, olivine NaFePO 4 is attractive due to its high theoretical capacity and large electrochemical stability window. In this work, AlF 3 -coated olivine NaFePO 4 is obtained by a facile electrochemical ion-exchange process in an aqueous electrolyte that is more environmentally friendly than conventional carbonate electrolytes. The robust AlF 3 -coated NaFePO 4 displays better cycling stability and surface stability in the electrochemically fragile aqueous electrolyte. Physicochemical and electrochemical measurements confirm that the optimal improvement in the electrochemical performance of NaFePO 4 is obtained using 0.5 wt% AlF 3 in an aqueous electrolyte solution. Furthermore, the AlF 3 coating on the surface of olivine NaFePO 4 reduces surface failure by stabilizing the NaFePO 4 surface in an aqueous electrolyte solution and facilitates lithium ion diffusion, leading to a NaFePO 4 material with lower surface resistance and better surface stability.